By Noah Veltman Twitter: @veltman Email: noah@noahveltman.com
The black lines indicate the territories each team would claim if everyone just cheered for the closest team.
Hover over a region to see the counties in which that team is actually the most popular team by number of Facebook likes.
Click on a region to keep it highlighted. Click again to un-highlight it. Use the links in the lower-left corner to highlight an entire division.
Thanks to Sean Taylor and the Facebook Data science team for the fan data. Read their post here.
Scroll down to read some observations. Territority maps for the NBA, MLB, NHL, and MLS are available here.
For more on how the hypothetical boundaries are calculated, read about Voronoi diagrams.
Mouse over or click on a region
 
Observations
State Lines
Fan bases that don't fall on closest-team lines often fall along state lines instead.
Most of Pennsylvania is Steelers country, even though large parts of the state are actually closer to Baltimore or Buffalo.
Mississippi and Alabama are almost entirely Saints country, even though large parts are closer to the Titans or Falcons.
The Titans, Falcons, Colts, Bears, and Lions fans also generally fall along home state lines.
Teams with a lot more fan territory than geography would predict
Cowboys - In terms of geographic reach, the Cowboys really are America's Team. Their fan empire stretches from Kern County, California to central Florida to southern Virginia to northern Montana.
Steelers - Another large fan empire that claims almost all of Pennsylvania and stretches across Appalachia and both Carolinas.
Packers - A fan base that includes lots of countries across the northern US, all the way to Oregon. (They also cover most of Alaska)
Bears - Most of Illinois is closer to St. Louis than Chicago, but the Bears fan base covers the whole state.
Saints - Their fan base covers all of Louisiana, Mississippi, and Alabama, at the expense of the Texans, Titans, and Falcons respectively.
Dolphins - Their fan base covers many Florida counties that are closer to Tampa Bay or Jacksonville.
Giants - Despite sharing a stadium with the Jets, they claim virtually all of the surrounding territory, with the exception of Nassau County.
Teams with a lot less fan territory than geography would predict
Texans - As a recent expansion team, their fan base only covers the Houston metro area. The rest of Texas goes for the Cowboys, and western Louisiana goes for the Saints.
Panthers - Their fan base covers the core of western North Carolina around Charlotte, but are surrounded by Steelers fans on all sides.
Jaguars - Their fan base only covers a small part of northeastern Florida. The Falcons get all of South Georgia and the Dolphins and Saints carve up the rest of the Panhandle.
Bills - They lose upstate New York to the Giants and northern Pennsylvania to Steelers.
Rams - They lose the majority of their geography-based territory, to the Cowboys in the south, to the Chiefs in the west, and to the Bears in the north.
Jets - They share a stadium with the Giants, but are only more popular than them in Nassau County.
Cardinals - They lose most of their geography-based territory to the Cowboys on all sides. Even southern Arizona is Cowboys country.
Chargers - Their fan base only extends as far north as Orange County. Most of the Inland Empire and Los Angeles area still belongs to the Raiders.
Distant outposts
The map shows a handful of anomalous counties that root for faraway teams. This may be due to small Facebook sample sizes in those counties.
The 49ers are the most popular team in Ziebach County, South Dakota.
The Vikings are the most popular team in Camden and Washington Counties, North Carolina.
The Giants are the most popular team in Osceola County, Florida.
The Lions are the most popular team in Polk County, Nebraska and Adams County, Idaho.
The Falcons are the most popular team in Fallon County, Montana.
The Bears are the most popular team in Alpine County, California.
The Patriots are the most popular team in Hampton County, South Carolina; Jones County, Iowa; and Charlotte and Flagler Counties, Florida
Data issues
Facebook likes are far from a perfect method for measuring NFL fandom. In sparsely-populated areas of the country, counties are likely to have a very small sample size. People who like things on Facebook are also not a perfect cross-section of football fans (they probably skew younger, for example). Other data sources that could be used as proxies for fan interest (but are subject to their own biases) are things like: home game attendance, merchandise sales, TV ratings, or volume of tweets about a team.
Additionally, the winner-take-all method of coloring counties lacks a sense of counties where a team dominates vs. counties where it is barely more popular than the runners-up. An alternate approach might be to use shading to convey the margin by which a team wins the county, but this makes it difficult to compare more than two teams at once, and there's still a tradeoff between coloring by percentage vs. number of fans.